Nickel base superalloys are widely used in gas turbine engines. Originally such alloys were used in conventionally cast form consisting of many randomly oriented equiaxed grains. Substantial property improvements were obtained by a casting technique known as directional solidification which was initially used to produce columnar grain articles consisting of a multiplicity of elongated oriented grains whose axis of elongation is constrained to be parallel to the axis of maximum stress. A subsequent refinement permits the production of single crystal articles and these articles represent the state-of-the-art in superalloy technology. The present invention concerns the improvement of the oxidation resistance of single crystal superalloy articles through the addition of small amounts of hafnium and/or silicon.
Silicon is known as a constituent of superalloys and is shown for example in U.S. Pat. Nos. 2,621,122, 2,994,605, 3,005,704, 3,411,898 and 3,524,744. Such minor additions have, to our knowedge, been made only to alloys intended for use in equaxed form. We are unaware that silicon has even been intentionally added to single crystal nickel base superalloy articles.
Hafnium has also been used in nickel base superalloy articles although to a lesser extent. U.S. Pat. No. 3,005,705 suggests the use of 0.1-1.0% hafnium in a equiaxed alloy article. The benefit attributed to hafnium in this patent is improved hgh temperature mechanical properties and it does not appear that any improvement in oxidation resistance was recognized. Hafnium has also been widely used in directional solidification columnar grained alloys where it provides improved transverse grain boundary ductility. This is described for example in U.S. Pat. No. 3,677,747. Again, in this patent there is no discussion of enhanced oxidation resistance. We are unaware that small hafnium additions have ever been made to single crystals for any purpose and in fact it was previously thought that hafnium should be avoided in single crystal articles as discussed in U.S. Pat. No. 4,116,723.